1. Field of the Invention
The present invention relates to an oscillator circuit having a temperature dependence, and more particularly, to an oscillator circuit that changes the period of an output signal depending on on the ambient temperature.
2. Description of Related Art
Recently, portable devices such as notebook-size personal computers are provided with a DRAM (Dynamic Random Access Memory) of large capacity so as to store therein a large amount of process data. In a DRAM, since the data retained by respective memory cells are being lost as time elapses, it is necessary to refresh the DRAM before the retained data are lost.
In a DRAM, due to an operating current at the time of refresh operation, a battery working as a power supply source is consumed. Generally, data retention property of a memory cell has a temperature dependency. That is, a lower chip temperature increases the data retention period, thereby raising the data retention capability of the DRAM. Furthermore, the chip temperature of a DRAM changes depending on the operational state of the DRAM itself, and the DRAM chip temperature is higher at the time of normal operation and is lower at the time of data retention. Accordingly, it is desired that the period of time between the time instant at which the refresh operation is carried out and the time instant at which the next refresh operation is carried out (refresh period) be elongated when the chip temperature is lower at the time of data retention. This is aimed to reduce the number of times of the refresh operation carried out during a predetermined period of time to lower the power dissipation of the DRAM caused by the refresh operation. Especially, the lower power dissipation is strongly required for portable devices provided with a DRAM. Therefore, it is strongly required that the power dissipation due to the refresh operations of a DRAM be lowered.
As a technique to change the refresh period depending on the ambient temperature, there is a known technique described in Jpn. Pat. Appln. Laid-Open Publication No. 2003-100074. Generally, in the technique generating the refresh period which changes depending on the ambient temperature, an oscillator circuit is used. As methods for changing the refresh period by using an oscillator circuit, there are known two types: a digital system and an analog system. Under the digital system, a frequency demultiplication circuit or a frequency divider is used which divides the frequency of an output signal from an oscillator circuit that outputs a signal of a constant period, and the frequency dividing ratio is changed based on the ambient temperature. Under the analog system, the period itself of an output signal from an oscillator circuit is controlled in the analog manner depending on the ambient temperature.
FIG. 13A shows a block diagram of a refresh period generation circuit 200 of digital system as described above, whereas FIG. 13B shows a graphical representation indicative of the relation between the period of an output signal of the refresh period generation circuit 200 and the ambient temperature. In FIG. 13B, graph (A) indicates required refresh period property refresh period performance), and graph (B) indicates the refresh period that the refresh period generation circuit 200 generates. An oscillator 202 oscillates at a constant period (fundamental period) without depending on the temperature change based on a constant current generated by a constant current source 201 having a property of no temperature dependency. A frequency demultiplication circuit 203 divides the frequency of an output signal from the oscillator 202 by a frequency dividing ratio determined based on the ambient temperature (chip temperature) that a temperature sensor 204 detects, and outputs a refresh period signal SigREF.
The refresh period required in a DRAM is generally inversely proportional to the ambient temperature, as shown by graph (A) in FIG. 13B. In the refresh period generation circuit 200, the frequency demultiplication circuit 203 sets the frequency dividing ratio at “1” (no frequency demultiplication) when the chip temperature sensed by the temperature sensor 204 is between 105° C. and 85° C., and outputs an output signal from the oscillator 202, that oscillates at the fundamental period T0, as a refresh period signal without changing the period, as shown by graph (B). The frequency demultiplication circuit 203 increases the frequency dividing ratio as the sensed chip temperature falls, and outputs refresh period signals specifying periods, which are two times, four times etc. the fundamental period by setting the frequency dividing ratio at “2”, “4”. . . . With the refresh period generation circuit 200 of digital system, a refresh period having a large magnification ratio to the fundamental period of the oscillator 202 can be obtained, and a refresh period signal SigREF specifying a period close to a required refresh period can be obtained.
FIG. 14A shows a block diagram of a refresh period generation circuit 200a of analog system, whereas FIG. 14B shows a graphical representation indicative of the relation between the period of an output signal of the refresh period generation circuit 200a and the ambient temperature. A current source 201a is so configured as to be able to increase or decrease the output current value depending on the ambient temperature. An oscillator 202a oscillates at a period depending on the ambient temperature based on the current generated by the current source 201a having the temperature dependency. A frequency demultiplication circuit 203a divides the frequency of an output signal from the oscillator 202a by a constant frequency dividing ratio, and outputs a refresh period signal SigREF. Employing this configuration, the refresh period generation circuit 200a of analog system outputs a refresh period that increases linearly as the ambient temperature falls, as shown by graph (B) in FIG. 14B.
In the refresh period generation circuit 200 of digital system, if the chip temperature sensed by the temperature sensor 204 fluctuates around the switching temperature for switching the frequency dividing ratio of the frequency demultiplication circuit 203, there is raised a problem that the period of the refresh period signal output from the frequency demultiplication circuit 203 largely fluctuates. Accordingly, the temperature sensor 204 is required to sense temperature with a higher accuracy.
For example, when the chip temperature is actually 50° C. and the temperature sensor 204 recognizes the chip temperature to be 45° C. or lower, the refresh period becomes eight times the fundamental period T0. As a result, the difference between the graph (A) and the graph (B) becomes small, as shown by a dotted line in FIG. 13B, and there is raised a problem that margin for the required refresh period becomes small. Conversely, when the chip temperature is actually 45° C. and the temperature sensor 204 recognizes the chip temperature to be 50° C., the refresh period becomes four times the fundamental period T0. As a result, the refresh period becomes unnecessarily short, thereby raising a problem of increased power dissipation.
On the other hand, in the refresh period generation circuit 200a of analog system, since the refresh period is changed in the analog manner in accordance with rise and fall of the current value of the current source 201a, even if the sensed temperature actually fluctuates, the refresh period does not sufficiently match the fluctuation of the sensed temperature, which situation is different from that employing the digital system. Accordingly, in the analog system, if it is desired that the refresh period be changed significantly, it is necessary to change the current value of the current source 201a widely. In this case, if the current value at a high temperature is set low so as to suppress the power dissipation of the oscillator 202a at the high temperature, the current value is excessively reduced at a lower temperature, thereby destabilizing the operation of the oscillator 202 at the lower temperature. Conversely, if the current value at a low temperature is set higher so as to secure the effective operation at the low temperature, there is raised a problem that the power dissipation is increased at a higher temperature. Thus, when employing the analog system, actually, the refresh period cannot be effectively changed, and the change ratio of the refresh period of the analog system is generally suppressed to approximately several times at a maximum. Accordingly, in the analog system, when the chip temperature is low, there is raised a problem that a large difference is caused between the required refresh period (graph (A) in FIG. 14B) and the refresh period specified by the refresh period signal SigREF (graph (B)) output from the frequency demultiplication circuit 203a. 